The long-term goal of the studies proposed is to understand the mechanism by which members of the tumor necrosis factor (TNF) and their receptors (TNFRs) regulate the healthy and pathological immune systems. TNF- and TNFR-like cytokine-receptor pairs are critical mediators for immune functions and homeostasis. Mutations in many of the genes within the family can lead to various forms of immune disorders in humans. Recent evidence shows that TNFR-like molecules exist as pre-formed complexes on the cell surface prior to ligand binding. This pre-association appears to allow only homo-specific complexes but not hetero-complexes. The TNF-like ligand TRAIL has been vaunted as a potential anti-tumor agent because of its selective cytotoxicity to tumor cells. TRAIL binds to four different cell surface receptors, some of which are signal-deficient and are therefore termed "decoys". The first part of this proposal concerns whether the TRAIL receptors can form heterocomplexes as a means to regulate their cytotoxic function. TNF can mediate multiple cellular responses including inflammation, cellular proliferation, differentiation and apoptosis/necrosis. TNF-Rl elicits these diverse responses by recruiting FADD, RIP and TRAF2 through a common adapter protein called TRADD. It is unclear if all of these signaling proteins can be present at the same receptor complex concomitantly. Perturbation of recruitment of these proteins may underlie the molecular cause for many TNFrelated diseases. The second part of this proposal attempts to elucidate the mechanism by which TNFR coordinates the recruitment of these signaling proteins.